The end is beginning for NASA's three aging space shuttles, with just five more missions on tap this year before the orbiter fleet retires in the fall.

That is, unless NASA needs a few more months to fly those remaining missions or President Barack Obama chooses to extend the shuttle program to fill a looming gap in U.S. human spaceflight capability.

Though the ultimate path forward for NASA has not yet been decided, the space agency is at a turning point after nearly 29 years of shuttle flight.

"Obviously it's the end of an era," said Roger Launius, space history curator at the Smithsonian's National Air and Space Museum. "There's a certain amount of nostalgia and a sense of loss, no question."

The very last space shuttle flight, the STS-133 mission of the shuttle Discovery to the International Space Station, is scheduled for September 2010. The launch will be the 134th shuttle voyage since the fleet's debut in 1981.

"It's starting to hit home, I have to admit to you," said NASA's shuttle launch director Mike Leinbach after the Nov. 16 liftoff of Atlantis on the STS-129 flight, the fifth and last shuttle trip of 2009. "After this one, there's one more scheduled for Atlantis, two more for each of the other vehicles."

Shuttle legacy
The shuttle has had incredible highs, and terrible lows, over its decades-long history since the launch of Columbia on STS-1 on April 12, 1981. Fourteen astronauts have been killed and two shuttles, Challenger and Columbia, lost during accidents.

"It had some very notable and public failures, and those are often what it's remembered for," Launius told SPACE.com. "The loss of the two vehicles with the crews was just tragic. But overall, it was a pretty successful program."

The space shuttle, officially NASA's Space Transportation System, was the first-ever reusable spacecraft. It consists of a payload bay-equipped orbiter to carry crew and cargo, with separate reusable solid rocket boosters to help it lift to space, and a disposable orange external tank to hold the chilled liquid fuel for its main engines.

"They built a reusable vehicle," Launius said. "That's pretty remarkable that they pulled that off. Nobody had ever done that before."

Slideshow: Month in Space: January 2014
But the space shuttle fleet hasn't achieved all its goals. Originally, NASA conceived it as a system that could fly frequent and inexpensive trips to space on almost an airline-like brisk schedule.

"It was supposed to be routine, safe and affordable, in addition to being highly capable. But it was never routine, [and] it was very expensive," said John Logsdon, a space policy expert and professor emeritus at George Washington University in St. Louis.

The shuttle's safety record was "decent, but not decent enough," he said. "It's riskier than we would like for a vehicle carrying people."

Nonetheless, it accomplished a lot, including the launch and multiple servicing trips of what's probably the world's best-known and loved observatory, the Hubble Space Telescope. And the shuttle has played a vital role in constructing the space station, the world's largest space laboratory and residence.

"The assembly of the space station could not have been done without the space shuttle, and the assembly of the space station is one of the great engineering achievements of mankind," said space shuttle program manager John Shannon. "So the space shuttle will have done a good job."

Of course, an unforgettable part of the space shuttle's legacy will always be its tragic accidents. On Jan. 28, 1986, the world watched stunned as the shuttle Challenger and its seven-member crew, including teacher Christa McAuliffe, were lost in a fiery explosion shortly after launch. And again on Feb. 1, 2003, disaster struck when the shuttle Columbia and its seven astronauts perished while re-entering the atmosphere during their descent back to Earth.

"You know, we lost seven astronauts, and that was awful, just devastating," Leinbach said of the Columbia tragedy. "But we also lost an orbiter. And it's hard to explain to people, that when we lost Columbia that was like losing a family member almost. It's almost that deep when you work on these machines day in and day out."

After each catastrophe, NASA took a break to investigate the failures, and was able to regroup and resume the shuttle program.

Getting so many missions off the ground is a tall order, but one that NASA has accomplished before — indeed, the agency launched five flights in 2009. The record for most shuttle launches in a single year (nine missions in all) was set back in 1985.

"In terms of next year, I think the teams are very well prepared," said Bill Gerstenmaier, NASA's associate administrator for space operations, after the final launch in 2009. "We're at the right pace, the tempo feels good, it doesn't feel rushed. The challenge will be to just stay focused, just take it one flight at a time."

These last shuttle missions are all slated to travel to the space station to deliver final rooms and experiments, and to drop off spare parts to keep it functioning beyond the shuttle's retirement.

After the shuttles are grounded, Russia's Soyuz spacecraft will be the only vehicle approved to carry humans to the station. NASA has said it plans to field its replacement craft for the shuttle, the Ares I rocket and the Orion crew capsule, by 2015.

But outside experts have said it will likely be later, sometime in 2017, when the new spacecraft will be ready to launch astronauts into space. An independent committee that reviewed NASA's plan to replace the shuttle fleet and return astronauts to the moon said last year that commercially built spacecraft may be able to help ease the coming gap in U.S. manned spaceflight capability.

While the future is uncertain, the year 2010 will be sure to be an eventful one for NASA, and could mark the end of the space shuttle era.

Shannon said that finale was likely to be bittersweet.

"I'm sure it will be emotional," he said. "But I suspect that it will not be sadness over the passing of that era, but happiness that we were a part of it."